This question comes up frequently. Here's the standard response
Sometimes the OG is so high that the yeast doesn't get started or can peeter out. Did it start vigorously and then stop? I had to take out about 3L and topped back up with water. It went off like crazy. I siphoned the 3L off into a 4L carlo rosi bottle and topped it with water and it took off too. I'll recombine the two volumes when it comes time to bottle so as not to lose any. That would be my advice.